Use of spike tires has been banned by law in Japan so as to prevent powder dust pollution resulting from the use of spike tires. As a result, studless tires are now used instead of the spike tires in cold regions. The studless tires can have better grip performance on ice and snow when they have a lower elastic modulus at low temperatures and have higher traction, for example. In particular, the braking force on ice is largely affected by the effective contact area between rubber and ice. There is therefore a demand for a vulcanized rubber composition that is flexible at low temperatures, which can provide a larger effective contact area.
Disadvantageously, decrease in only the hardness of a vulcanized rubber composition by, for example, increasing the oil amount causes poor handling stability.
Generally, tread rubber of studless tires is often produced from natural rubber or butadiene rubber as a main ingredient (for example, see Patent Document 1). This is because these rubbers have a low glass transition temperature and flexibility, as well as high strength. However, natural rubber and butadiene rubber suffer reversion upon sulfur vulcanization. This phenomenon causes degradation of the rubber or deterioration of the cross-linking state of the rubber, and thereby the elastic modulus at low temperatures of the rubber is decreased. Further, the inventors of the present invention have found from their studies that the phenomenon causes excessive decrease in the hardness of the rubber, resulting in poor handling stability and poor abrasion resistance. In addition, reversion may excessively increase tan δ at high temperatures, which may cause poor fuel economy.
In some cases, tires, not limited to studless tires, are vulcanized at higher temperatures so as to be produced at higher productivity. In such cases, the aforementioned phenomenon more remarkably occurs. Accordingly, there is an additional problem of decrease in abrasion resistance caused by the reversion.
Conventionally, vulcanizable rubber compositions used in rubber products such as tires are prevented from suffering reversion and are allowed to have better heat resistance in the case of containing a vulcanization accelerator in a proportion higher than that of sulfur (a vulcanizing agent), or containing a thiuram vulcanization accelerator as a vulcanization accelerator, for example. Further, rubber compositions are known to be prevented from suffering reversion in the case of containing a cross-linking agent that is capable of forming a long chain cross-linking structure represented by formulas such as —(CH2)6—S—. Examples of the cross-linking agent include PERKALINK 900 and Duralink HTS (each produced by Flexsys), and Vulcuren KA9188 (produced by Bayer AG). These techniques effectively inhibit the reversion of natural rubber or isoprene rubber; however, the techniques disadvantageously have a less or no inhibiting effect on the reversion of butadiene rubber.
Patent Document 1: JP 2007-169500 A